The prophylactic and pre-emptive use of ganciclovir (GCV) both reduce significantly the incidence of CMV disease after sibling BMT but it is unclear which of these strategies is best for volunteer unrelated donor (VUD) BMT patients. We reviewed 49 consecutive patients, who received a T-depleted VUD BMT (from March 1990 to March 1996) for the treatment of CML in chronic phase, and were CMV seropositive before transplant or had a CMV seropositive donor. Patients were conditioned with cyclophosphamide (120 mg/kg for 2 days) and total body irradiation (13.2-14.4 Gy). Prophylaxis for GVHD was cyclosporin A and methotrexate with ex vivo or in vivo T cell depletion. Twenty-seven patients received pre-emptive GCV if CMV infection was detected by short-term culture before day +120 post BMT. Twenty-two patients received prophylactic GCV from engraftment until day +120 post BMT. The probabilities of CMV infection and disease occurring by 1 year post-BMT were greater in the pre-emptive GCV group than in the prophylactic GCV group (73.8% and 64.0% vs 53.1% and 30.0%, respectively; P=0.04 and 0.07). The incidence of death from CMV disease was similar in both groups (3/12 (25%) vs 3/10 (30%), respectively) and there was no difference in 1 year survival (55.6% vs 54.2%, respectively). New strategies are urgently required for the prevention of CMV disease after T-depleted VUD BMT.